Thousands of people escaped to their rooftops to avoid drowning in the deadly floodwaters. Many were rescued in the days that followed, in some cases by journalists or good samaritans with fishing boats. Tens of thousands of Texas residents, left with nowhere to go, have retreated to temporary shelters.

“We are expecting over 450,000 potential registers of disaster victims. That is a huge number,” Brock Long, who leads the US Federal Emergency Management Agency, said in a statement Monday morning.

Harvey made landfall on Friday as a Category 4 hurricane, a second landfall as a tropical storm on Wednesday, and has since been downgraded to a tropical depression.

Harris County, Texas – home to Houston and about 5 million people – was a region among the hardest-hit by the storm’s flooding. As the the waters begin to recede, many are wondering why mandatory or even voluntary evacuation orders weren’t issued for the region ahead of the unprecedented storm.

“We always say, ‘Run from water, hide from wind.’ When we say that, we mean storm surge, not rain,” Harris County Judge Ed Emmett told reporters during a press conference on August 25. “In this case, we’ll have a lot of water, but it’s not the kind of water that we would ask people to evacuate from.”

The biggest reason Houston officials didn’t tell residents to evacuate was to avoid clogging highways and other roads at dangerous levels. When Hurricane Rita barreled toward Texas in 2005, for example, an exodus of about 3 million people contributed to at least 73 deaths – though some have estimated as many as 107 – before the storm.

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Before and after the floods from Harvey dumped over 2 feet of rain on Houston in three days.

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Shutterstock/Reuters/Business Insider

“Traffic jams stretched across hundreds of miles over two days, and many people ran out of gas,” reporters Jim Malewitz and Brandon Formby wrote in The Texas Tribune. “Dozens died from accidents and heat-related illnesses, all before Rita even made landfall.”

Had Harris County issued an evacuation order even several days in advance, a similar backup may have ensued – and it could have happened on roads that quickly got flooded with several feet of fast-moving water.

As The Tribune and other outlets have reported, most flood deaths (about two-thirds) happen in vehicles. This is because many people drive into what appears to be shallow water on a roadway, only to be swept away by deceptively strong currents and deeper-than-expected flooding.

Michael Lowry, a scientist with the University Corporation for Atmospheric Research, underscored this point in a tweet on Sunday.

In a press conference on Sunday morning, Houston Mayor Sylvester Turner defended his decision not to evacuate residents en masse.

“You literally cannot put 6.5 million people on the road,” Turner said, according to CNN. “If you think the situation right now is bad, you give an order to evacuate? You are creating a nightmare.”

Officials in Harris County did ultimately issue evacuation orders in some areas early Monday morning. But by that point, many people weren’t able to comply with them.

Aisha Nelson, who moved to Houston after Hurricane Katrina devastated her home, talked about her situation from a rooftop in a “Good Morning America” live broadcast Monday morning. She said about 30 other people were on the roof with her.

“It’s not going good for us. Across the street, the building is caving in, and it’s water everywhere,” Nelson said. “We tried to leave, but there was nowhere for us to go … Please help us. I’m scared.” (One of the show’s hosts said the program notified the US Coast Guard about Nelson’s location.)